May 10 (GMM) The issue of 'fuel flow' has returned to the formula one paddock.

A few races ago, we reported that the FIA was clamping down on monitoring the maximum allowed fuel flow limit, amid suspicions top manufacturers may have found a loophole.

It was suspected some teams - perhaps Ferrari and Mercedes - had found a way to increase fuel flow at certain moments whilst always ensuring the FIA's fuel flow meter was not alerted.

Fascinatingly, as the controversy returned to the agenda in Barcelona, Mercedes' team boss Toto Wolff admitted to Stuttgarter Nachrichten newspaper that Lewis Hamilton and Nico Rosberg have at their disposal an "additional mode" to deploy in qualifying.

But Niki Lauda, Mercedes' team chairman, says he has heard other "rumours".

"There are rumours that some teams have an extra (fuel) tank, and once it fills up it to the right amount can be used in the race," he told German television RTL.

Lauda might be referring to the fact that while Ferrari normally trails the Mercedes in qualifying, the red cars are often much closer to the ultimate pace in races.

F1's governing body said in a technical directive issued to teams in Barcelona that, with "immediate effect", fuel pressure must "remain constant".

"We will also be carrying out careful physical inspections of all fuel systems in use," race director Charlie Whiting added.

As for Lauda's claims of an 'extra tank', the F1 legend and triple world champion added: "These are all rumours, but what is certainly true is that from now on it no longer happens."